This invention relates to a membrane device which is an improvement on a frameless array of hollow fiber membranes and a method of maintaining clean fiber surfaces while filtering a substrate to withdraw a permeate, which is also the subject of U.S. Pat. No. 5,248,424; and, to a method of forming a header for a skein of fibers. The term “vertical skein” in the title (hereafter “skein” for brevity), specifically refers to an integrated combination of structural elements including (i) a multiplicity of vertical fibers of substantially equal length; (ii) a pair of headers in each of which are potted the opposed terminal portions of the fibers so as to leave their ends open; and, (iii) permeate collection means held peripherally in fluid tight engagement with each header so as to collect permeate from the ends of the fibers.
The term “fibers” is used for brevity, to refer to “hollow fiber membranes” of porous or semipermeable material in the form of a capillary tube or hollow fiber. The term “substrate” refers to a multicomponent liquid feed. A “multicomponent liquid feed” in this art refers, for example, to fruit juices to be clarified or concentrated; wastewater or water containing particulate matter; proteinaceous liquid dairy products such as cheese whey, and the like. The term “particulate matter” is used to refer to micron-sized (from 1 to about 44 μm) and sub-micron sized (from about 0.1 μm to 1 μm) filterable matter which includes not only particulate inorganic matter, but also dead and live biologically active microorganisms, colloidal dispersions, solutions of large organic molecules such as fulvic acid and humic acid, and oil emulsions.
The term header is used to specify a solid body in which one of the terminal end portions of each one of a multiplicity of fibers in the skein, is sealingly secured to preclude substrate from contaminating the permeate in the lumens of the fibers. Typically, a header is a continuous, generally rectangular parallelpiped of solid resin (thermoplastic or thermosetting) of arbitrary dimensions formed from a natural or synthetic resinous material. In the novel method described hereinbelow, the end portions of individual fibers are potted in spaced-apart relationship in cured resin, most preferably by “potting” the end portions sequentially in at least two steps, using first and second potting materials. The second potting material (referred to as “fixing material”) is solidified or cured after it is deposited upon a “fugitive header” (so termed because it is removable) formed by solidifying the first liquid. Upon removing the fugitive header, what is left is the “finished” or “final” header formed by the second potting material. Of course, less preferably, any prior art method may be used for forming finished headers in which opposed terminal end portions of fibers in a stack of arrays are secured in proximately spaced-apart relationship with each other.
The '424 patent required potting the opposed ends of a frameless array of fibers and dispensed with the shell of a module; it was an improvement on two preceding configurations disclosed in U.S. Pat. Nos. 5,182,019, and 5,104,535, each of which used frameless arrays and avoided potting the fibers. The efficiency of gas-scrubbing a '424 array was believed to be due, at least in large part, to a substantial portion of the fibers of the fibers in the array lying in transverse relationship to a mass of rising bubbles, referred to herein as a “column of rising bubbles”, so as to intercept the bubbles. Specific examples are illustrated in FIGS. 9, 9A, 10 and 11 of the '424 patent.
A '424 “array” referred to a bundle of arcuate fibers the geometry of which array was defined by the position of a pair of transversely spaced headers in which the fibers were potted. In the '424 array, as in the array of this invention, each fiber is free to move independently of the others, but the degree of movement in the '424 is unspecified and arbitrary, while in the vertical skein of this invention, movement is critically restricted by the defined length of the fibers between opposed headers. Except for their opposed ends being potted, there is no physical restraint on the fibers of a skein. To avoid confusion with the term “array” as used for the '424 bundle of arcuate fibers, the term “skein fibers” is used herein to refer to plural arrays. An “array” in this invention refers to plural, essentially vertical fibers of substantially equal lengths, the one ends of each of which fibers are closely spaced-apart, either linearly in the transverse (y-axis herein) direction to provide at least one row, and typically plural rows of equidistantly spaced apart fibers. Less preferably, a multiplicity of fibers may be spaced in a random pattern. Typically, plural arrays are potted in a header and enter its face in a generally x-y plane (see FIG. 5). The width of a rectangular parallelpiped header is measured along the x-axis, and is the relatively shorter dimension of the rectangular upper surface of the header; and, the header's length, which is its relatively longer dimension, is measured along the y-axis.
This invention is particularly directed to relatively large systems for the microfiltration of liquids, and capitalizes on the simplicity and effectiveness of a configuration which dispenses with forming a module in which the fibers are confined. As in the '424 patent, the novel configuration efficiently uses a cleansing gas, typically air, discharged near the base of a skein to produce bubbles in a specified size range, and in an amount large enough to scrub the fibers, and to cause the fibers to scrub themselves against one another. Unlike in the '424 system, the fibers in a skein are vertical and do not present an arcuate configuration above a horizontal plane through the horizontal center-line of a header. As a result, the path of the rising bubbles is generally parallel to the fibers and is not crossed by the fibers of a vertical skein. Yet the bubbles scrub the fibers. The restrictedly swayable fibers, because of their defined length, do not get entangled, and do not abrade each other excessively, as is likely in the '424 array. The defined length of the fibers herein minimizes (i) shearing forces where the upper fibers are held in the upper header, (ii) excessive rotation of the upper portion of the fibers, as well as (iii) excessive abrasion between fibers. The fibers of this invention are confined so as to sway in a “zone of confinement” (or “bubble zone”) through which bubbles rise along the outer surfaces of the fibers. The side-to-side displacement of an intermediate portion of each fiber within the bubble zone is restricted by the fiber's length. The bubble zone, in turn, is determined by one or more columns of vertically rising gas bubbles, preferably of air, generated near the base of a skein.
Since there is no module in the conventional sense, the main physical considerations which affect the operation of a vertical skein in a reservoir of substrate relate to intrinsic considerations, namely, (a) the fiber chosen, (b) the amount of air used, and (c) the substrate to be filtered. Such considerations include the permeability and rejection properties of the fiber, the process flow conditions of substrate such as pressure, rate of flow across the fibers, temperature, etc., the physical and chemical properties of the substrate and its components, the relative directions of flow of the substrate (if it is flowing) and permeate, the thoroughness of contact of the substrate with the outer surfaces of the fibers, and still other parameters, each of which has a direct effect on the efficiency of the skein. The goal is to filter a slow moving or captive substrate in a large container under ambient or elevated pressure, but preferably under essentially ambient pressure, and to maximize the efficiency of a skein which does so (filters) practically and economically.
In the skein of this invention, all fibers in the plural rows of fibers, staggered or not, rise generally vertically while fixedly held near their opposed terminal portions in a pair of opposed, substantially identical headers to form the skein of substantially parallel, vertical fibers. This skein typically includes a multiplicity of fibers, the opposed ends of which are potted in closely-spaced-apart profusion and bound by potting resin, assuring a fluid-tight circumferential seal around each fiber in the header and presenting a peripheral boundary around the outermost peripheries of the outermost fibers. The position of one fiber relative to another in a skein is not critical, so long as all fibers are substantially codirectional through one face of each header, open ends of the fibers emerge from the opposed other face of each header, and substantially no terminal end portions of fibers are in fiber-to-fiber contact. We found that the skein of fibers, deployed to be restrictedly swayable, were as ruggedly durable as they were reliable in operation.
The fibers are stated to be “restrictedly swayable”, because the extent to which they may sway is determined by the free length of the fibers relative to the fixedly spaced-apart headers, and the turbulence of the substrate. When a large number of fibers is used in a skein, as is typically the case herein, the movement of a fiber adjacent to others may be modulated by the movement of the others, but the movement of fibers within a skein is constricted. This system is therefore limited to the use of a skein of fibers having a critically defined length relative to the vertical distance between headers of the skein. The defined length limits the side-to-side movement of the fibers in the substrate in which they are deployed, except near the headers where there is negligible movement.
In the prior art, a vertical skein of fibers in a substrate is typically avoided due to expected problems relating to channelling of the feed. However, because the fibers are restrictedly swayable in a “bubble zone” as described herebelow, the fibers are substantially evenly contacted over their individual surfaces with substrate and provide filtration performance based on a maximized surface which is substantially the sum of the surface areas of all fibers in contact with the substrate. Moreover, because of the ease with which the substrate coats the surfaces of the vertical fibers in a skein, and the accessibility of those surfaces by air bubbles, the fibers may be densely arranged in a header to provide a large membrane surface of up to 1000 m2 and more.
One header of a skein is displaceable in any direction relative to the other, either longitudinally (x-axis) or transversely (y-axis), only prior to the headers being vertically fixed in spaced apart parallel relationship within a reservoir, for example, by mounting one header above another, against a vertical wall of the reservoir which functions as a spacer means. This is also true prior to spacing one header above another with other spacer means such as bars, rods, struts, I-beams, channels, and the like, to assemble plural skeins into a “bank of skeins” (“bank” for brevity), in which bank a row of lower headers is directly beneath a row of upper headers. After assembly into a bank, a segment intermediate the potted ends of each individual fiber is displaceable along either the x- or the y-axis, because the fibers are loosely held in the skein. There is essentially no tension on each fiber because the opposed faces of the headers are spaced apart at a distance less than the length of an individual fiber.
By operating at ambient pressure, mounting the headers of the skein within a reservoir of substrate, and by allowing the fibers restricted movement within the bubble zone in a substrate, we minimize damage to the fibers. Because, a header secures at least 10, preferably from 50 to 50,000 fibers, each generally at least 0.5 m long, in a skein, it provides a high surface area for filtration of the substrate.
The fibers divide a reservoir into a “feed zone” and a withdrawal zone referred to as a “permeate zone”. The feed of substrate is introduced externally (referred to as “outside-in” flow) of the fibers, and resolved into “permeate” and “concentrate” streams. The skein, or a bank of skeins of this invention is most preferably used for microfiltration with “outside-in” flow. Typically a bank is used in a relatively large reservoir having a volume in excess of 10 L (liters), preferably in excess of 1000 L, such as a flowing stream, more typically a reservoir (pond or tank). Most typically, a bank or plural banks with collection means for the permeate, are mounted in a tank under atmospheric pressure, and permeate is withdrawn from the tank.
Where a bank or plural banks of skeins are placed within a tank or bioreactor, and no liquid other than the permeate is removed the tank is referred to as a “dead end tank”. Alternatively, a bank or plural banks may be placed within a bioreactor, permeate removed, and sludge disposed of; or, in a tank or clarifier used in conjunction with a bioreactor, permeate removed, and sludge disposed of.
Operation of the system relies upon positioning at least one skein, preferably a bank, close to a source of sufficient air or gas to maintain a desirable flux, and, to enable permeate to be collected from at least one header. A desirable flux is obtained, and provides the appropriate transmembrane pressure differential of the fibers under operating process conditions. “Transmembrane pressure differential” refers to the pressure difference across a membrane wall, resulting from the process conditions under which the membrane is operating.
The relationship of flux to permeability and transmembrane pressure differential is set forth by the equation:J=k▴Pwherein, J=flux; k=permeability constant;▴P=transmembrane pressure differential; and k=1/μRm where μ=viscosity of water and, Rm=membrane resistance.
The transmembrane pressure differential is preferably generated with a conventional non-vacuum pump if the transmembrane pressure differential is sufficiently low in the range from 0.7 kPa (0.1 psi) to 101 kPa (1 bar), provided the pump generates the requisite suction. The term “non-vacuum pump” refers to a pump which generates a net suction side pressure difference, or, net positive suction head (NPSH), adequate to provide the transmembrane pressure differential generated under the operating conditions. By “vacuum pump” we refer to one capable of generating a suction of at least 75 cm of Hg. A pump which, generates minimal suction may be used if an adequate “liquid head” is provided between the surface of the substrate and the point at which permeate is withdrawn; or, by using a pump, not a vacuum pump. A non-vacuum pump may be a centrifugal, rotary, crossflow, flow-through, or other type. Moreover, as explained in greater detail below, once the permeate flow is induced by a pump, the pump may not be necessary, the permeate continuing to flow under a “siphoning effect”. Clearly, operating with fibers subjected to a transmembrane pressure differential in the range up to 101 kPa (14.7 psi), a non-vacuum pump will provide adequate service in a reservoir which is not pressurized; and, in the range from 101 kPa to about 345 kPa (50 psi), by superatmospheric pressure generated by a high liquid head, or, by a pressurized reservoir.
The fibers are not required to be subjected to a narrowly critical transmembrane pressure differential though fibers which operate under a small transmembrane pressure differential are preferred. A fiber which operates under a small transmembrane pressure differential in the range from about 0.7 kPa (0.1 psi) to about 70 kPa (10 psi) may produce permeate under gravity alone, if appropriately positioned relative to the location where the permeate is withdrawn. In the range from 3.5 kPa (0.5 psi) to about 206 kPa (30 psi) a relatively high liquid head may be provided with a pressurized vessel. The longer the fiber, which greater the area and the more the permeate.
In the specific instance where a bank is used in combination with a source of cleansing gas such as air, both to scrub the fibers and to oxygenate a mixed liquor substrate, most, if not all of the air required, is introduced either continuously or intermittently, near the base of the fibers near the lower header. The perforations through which the gas is discharged near the header are located close enough to the fibers so as to provide columns of relatively large bubbles, preferably larger than about 1 mm in nominal diameter, which codirectionally contact the fibers and flow vertically along their outer surfaces, scrubbing them. The outer periphery of the columns of bubbles define the zone of confinement in which the scrubbing force exerted by the bubbles on the fibers, keeps their surfaces sufficiently free of attached microorganisms and deposits of inanimate particles to provide a relatively high and stable flow of permeate over many weeks, if not months of operation. The significance of this improvement will be better appreciated when it is realized that the surfaces of fibers in conventional modules are cleaned nearly every day, and sometimes more often.
Because this system, like the '424 system, does away with using a shell, there is no void space within a shell to be packed with fibers; and, because of gas being introduced proximately to, and near the base of skein fibers, there is no need to maintain a high substrate velocity across the surface of the fibers to keep the surfaces of the fibers clean. As a result, there is virtually no limit to the number of restrictedly swayable fibers which may be used in a skein, the practical limit being set by (i) the ability to pot the ends of the fibers reliably; (ii) the ability to provide sufficient air to the surfaces of essentially all the fibers, and (iii) the number of banks which may be deployed in a tank, pond or lake, the number to be determined by the size of the body of water, the rate at which permeate is to be withdrawn, and, the cost of doing so.
Typically, a relatively large number of long fibers, at least 100, is used in a skein of restrictedly swayable fibers, the fibers operate under a relatively low transmembrane pressure differential and permeate is withdrawn with a non-vacuum pump. If the liquid head, measured as the vertical distance between the level of substrate and the level from which permeate is to be withdrawn, is greater than the transmembrane pressure differential under which the fiber operates, the permeate will be separated from the remaining substrate, due to gravity.
Irrespective of whether a non-vacuum pump, vacuum pump, or other type of pump is used, or permeate is withdrawn with a siphoning effect, it is essential that the fibers in a skein be positioned in a generally vertical attitude, rising above the lower header. An understanding of how a vertical skein operates will make it apparent that, since fibers in a skein are anchored at the base of the skein by the lower header, the specific gravity of the fibers relative to that of the substrate is immaterial and will not affect their vertical disposition.
The unique method of forming a header disclosed herein allows one to position a large number of fibers, in closely-spaced apart relationship, randomly relative to one another, or, in a chosen geometric pattern, within each header of synthetic resinous material. It is preferred to position the fibers in arrays before they are potted to ensure that the fibers are spaced apart from each other precisely, and, to avoid wasting space on the face of a header; it is essential, for greatest reliability, that the fibers not be contiguous. By sequentially potting the terminal portions of fibers in stages as described herein, the fibers may be cut to length in an array, either after, or prior to being potted. The use of a razor-sharp knife, or scissors, or other cutting means to do so, does not decrease the open cross-sectional area of the fibers' bores (“lumens”). The solid resin forms a circumferential seal around the exterior terminal portions of each of the fibers, open ends of which protrude through the permeate-discharging face of each header, referred to as the “aft” face.
Further, one does not have to cope with the geometry of a frame, the specific function of which is to hold fibers in a particular arrangement within the frame. In a skein, the sole function of the header spacing means is to maintain a fixed vertical distance between headers which are not otherwise spaced apart. In a skein of this invention, there is no frame.
The skein of this invention is most preferably used to treat wastewater in combination with a source of an oxygen-containing gas which is bubbled within the substrate, near the base of a lower header, either within a skein or between adjacent skeins in a bank, for the specific purpose of scrubbing the fibers and oxygenating the mixed liquor in activated sludge, such as is generated in the bioremediation of wastewater. It was found that, as long as enough air is introduced near the base of each lower header to keep the fibers awash in bubbles, and the fibers are restrictedly swayable in the activated sludge, a build-up of growth of microbes on the surfaces of the fibers is inhibited while permeate is directly withdrawn from activated sludge, and excellent flow of permeate is maintained over a long period. Because essentially all surface portions of the fibers are contacted by successive bubbles as they rise, whether the air is supplied continuously or intermittently, the fibers are said to be “awash in bubbles.”
The use of an array of fibers in the direct treatment of activated sludge in a bioreactor, is described in an article titled “Direct Solid-liquid Separation Using Hollow Fiber Membrane in an Activated Sludge Aeration Tank” by Kazuo Yamamoto et al in Wat. Sci. Tech. Vol. 21, Brighton pp 43-54, 1989, and discussed in the '424 patent, the disclosure of which is incorporated by reference thereto as if fully set forth herein. The relatively poor performance obtained by Yamamoto et al was mainly due to the fact that they did not realize the critical importance of maintaining flux by aerating a skein of fibers from within and beneath the skein. They did not realize the necessity of thoroughly scrubbing substantially the entire surfaces of the fibers by flowing bubbles through the skein to keep the fibers awash in bubbles. This requirement becomes more pronounced as the number of fibers in the skein increases.
As will presently be evident, since most substrates are contaminated with micron and submicron size particulate material, both organic and inorganic, the surfaces of the fibers in any practical membrane device must be maintained in a clean condition to obtain a desirable specific flux. To do this, the most preferred use of the skein as a membrane device is in a bank, in combination with a gas-distribution means, which is typically used to distribute air, or oxygen-enriched air between the fibers, from within the skein, or between adjacent skeins, at the bases thereof.
Tests using the device of Yamamoto et al indicate that when the air is provided outside the skein the flux decreases much faster over a period of as little as 50 hr, confirming the results obtained by them. This is evident in FIG. 1 described in greater detail below, in which the graphs show results obtained by Yamamoto et al, and the '424 array, as well as those with the vertical skein, all three assemblies using essentially identical fibers, under essentially identical conditions.
The investigation of Yamamoto et al with downwardly suspended fibers was continued and recent developments were reported in an article titled “Organic Stabilization and Nitrogen Removal in Membrane Separation Bioreactor for Domestic Wastewater Treatment” by C. Chiemchaisri et al delivered in a talk to the Conference on Membrane Technology in Wastewater Management, in Cape Town, South Africa, Mar. 2-5, 1992, also discussed in the '424 patent. The fibers were suspended downwardly and highly turbulent flow of water in alternate directions, was essential.
It is evident that the disclosure in either the Yamamoto et al or the Chiemchaisri et al reference indicated that the flow of air across the surfaces of the suspended fibers did little or nothing to inhibit the attachment of microorganisms from the substrate.